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Meet the turtles surviving an invasion of enormous tractors

On the trail of turtles

Nathan T. Simmons

By Wendy Mitman Clarke in Maryland

Crouching in the woods, amid a tangle of fallen trees and brush, greenbrier, probably poison ivy and who knows what else, I am acutely aware of two things: sweat is actually a state of being, and cicadas are insanely loud. Suddenly, a new sound grabs my attention, and beside me, Aaron Krochmal holds up a hand like a ranger on recon.

From the receiver slung around his neck, a rhythmic beeping signals that a radio transmitter is being picked up. Initially, it can be heard crackling with static, but it grows steadily clearer.

There’s a turtle on the move. Not a giant Galapagos tortoise. Not even a monster snapping turtle that, around here in the woods of Maryland’s eastern shore, has been known to reach manhole-cover size.

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No, what emerges, slogging stoically through the bracken and vine, is an eastern painted turtle, maybe 12 centimetres long, the bright yellow slashes down its neck and red-edged carapace a moving artwork. This particular animal has trudged this same path, forded the same creeks, clambered through these vines and over these fallen trees, every year for at least a decade or so, unwavering.

“How do they know? How do they learn how?” I whisper to Krochmal, a biologist at Washington College in Chestertown, Maryland. Since 2009, he and his colleague Timothy Roth at Franklin & Marshall College in Lancaster, Pennsylvania – along with dozens of summer research undergraduates at Washington College – have studied the navigation and spatial learning capabilities of these humble reptiles.

He’s about to answer when we, the transmitter, and even the cicadas are drowned out by the roar of a massive tractor rumbling to life, preparing to work the soybean field just beyond our little slice of wilderness.

Learning to navigate

We are at DuPont Chesapeake Farms, 3300 acres of agribusiness land mingled with forests, streams, wetlands and fields. Among its missions is serving as a “flagship of environmental stewardship for crop protection by investigating, demonstrating, and promoting sustainable practices in agriculture and wildlife management”.

That Krochmal and his team have been able to conduct groundbreaking research on turtle ecology and behaviour in a gigantic farming complex might seem counterintuitive. But it’s in part because this habitat lies right at the heart of a corporate farming facility that the research is possible and vital, says Krochmal.

“Human-modified landscapes are the new normal for the vast majority of the world’s species,” he says. “Working in an agricultural matrix allows us to study how animals adapt to and learn to navigate in response to human disturbance.”

Shane Brill

Krochmal and Roth’s early findings suggested that turtles struggle with those changes. Or at least, some of them do. They discovered that turtles unfailingly follow specific, intricate paths to distant water sources year in, year out. Wondering whether this behaviour was learned or innate, they introduced non-resident turtles to the farm. New turtles under the age of 4 years learned to travel the complex paths of the new environment just as precisely as experienced local turtles, whereas the new adults could not.

Follow-up experiments supported the idea. Krochmal and Roth gave turtles a drug to block the receptors that bind acetylcholine, a neurotransmitter associated with spatial memory. This disrupted the navigation of adults familiar with a particular route, but had no impact on new juveniles that hadn’t yet laid down those navigational memories.

Spatial memories

The work is ongoing: as I join Krochmal and Roth, they are looking into what environmental cues juvenile turtles use to form spatial memories of navigation, and how these memories are modified by future experiences. As the research continues, it is becoming increasingly clear that animal migrations may be driven, at least in part, by cognitive processes rather than being entirely down to instinct, raising new questions about the evolutionary origin of complex cognition.

Just as importantly, there are implications for conservation strategies. Habitat loss can lead to translocation, in which conservationists capture animals and move them to a similar environment in a different region in the hope that they will thrive there. The research suggests that for these particular turtles to survive such a move, they would have to be under the age of 4 years – and there may be an age limit on successful translocation for other animal species too.

Next to us, the roar of the tractor recedes – as does the receiver’s beeping. The turtle vanishes into the woods it knows so well. It might be too set in its ways to adapt if those woods were ever to change – but we now know that the younger turtles hidden in the undergrowth might be able to take such changes in their (very slow) stride.